Intro
In full disclosure this case was not one I contributed to in any way, unlike all the others I’ve reported on. Nevertheless, source who did work on this case told me sufficient details and it is an interesting case.
The setup
For this case to make any sense, you need to understand the background. If I got it right, some people were trying to restore a backup version of Windows 11 Professional. When they did this restore, they found the problem that they were not pciking up an IP address via dhcp if they were on a company network. If they did the restore while on a home office network it went OK.
So imagine the comlpexity in a modern IT environment this presents. You have the PV vendor, HP, the OS vendor, Microsoft, the dhcp service operator, in-house, the LAN service provider and the network gear vendor, Cisco. The fault could lie anywhere. They all initially claim their stuff is working fine (which is always the default statement) and look elsewhere.
So what I like to say is that any hypothesis is unlikely, yet one of them will prove to be correct, eventually.
More details
Packet traces showed the DHCP Discover request being sent by the PC, but not arriving to the DHCP server. Ah, you say, simple: the switch is guilty here of dropping the DHCP Discover packet, fix it. After all, “eating” dhcp packets is something misconfigured switches do all the time if dhcp snooping is misconfigured.
Yet the LAN service provider says the switch isn’t misconfigured. So they have to open a case with the switch vendor to understand the drop. I’m not sure where that support case went, meanwhile…
The in-house expert troubleshooters were able to take a second trace from a PC which did pick up an IP address after a restore. This restore feature of course used to work when it was initially released.